Process for removing finely divided solids from raw low temperature carbonization coal tars



.. q 1956 M. D. KULIK 2,774,716

PROCESS FOR REMOVING FINEILY DIVIDED souns FROM RAM LOW TEMPERATURECARBQNIZATION COAL TARS Filed Jan. 29, 1954 LT.O|L

Rw A TANK 24 2e 34 @IB 33 LHQUOR STRIPPING COLUMN 3o commuous INVENTORMETRO D. KULIK ATTORNEY United States Patent O" Metro D. Kulik,Pittsburgh, Pa., assignor to Pittsburgh Consolidation Coal Company,Pittsburgh, Pa., a corporation of Pennsylvania Application January 29,1954, Serial No. 406,926

11 Claims. (Cl. 196 14.1)

The present invention relates to a process for separating finely dividedsolids from low temperature carbonization coal 'tars, and moreparticularly to a process for effecting this separation by agglomeratingthe solids with a portion of the tar itself.

The present application is a continuation-in-part of copending U. S.application S. N. 260,265, filed by me on December 6, 1951, nowabandoned, and entitled Process for Removing Finely Divided Solids fromRaw Low Temperature Carbonization Coal Tars.

In continuous processes for carbonizing coal at low temperature, the tarproduct often contains a high concentration of finely divided particlesof coal or partially devolatilized solid fuel. The presence of theseparticles in the product tar is occasioned by entrainment of'theparticles in the efiluent gas and tar vapors from the carbonizationprocess. Some low temperature tars will contain 15 to 25 percent byweight of solids. Frequently more than 50 percent by weight of thesesolids will pass through a 325 mesh U. S. standard screen. Because ofthe very fine consistency of thesesolids, they will not settle out ofthe raw tar onjstanding. fMoreover, their removal from the tar byfiltration is impractical since the particles instantly clog theinterstices of any conventional filtering equipment. The high viscosityof the tar and the hard sediment makes continuous centrifuging'inefiective for the removalof'these solids.

The presence of these solids in the tar seriously reduces the value ofthe tar. Conventional processing of the solids laden raw tar forrecovery of valuable tar acids and oils by such methods as distillationor extraction is difficult. Moreover if the solids laden tar is refined,the solids become even more'concentrated infthe heavy pitch remainingafter the lighter, morevaluable constitutents have' been removed. Hencethe' ash content of the residual pitch increases and "its valuecorrespondingly decreases,

The primary object of this invention is to recover a 7 By the term coalI comprehend all ranks of coail,in--

eluding bituminous, sub-bituminous, brown coal,lign ite', 'andthelike. si V. According to the present process, raw tar containing finely dividedsolids is agitated vwith .a mixture: of a predominantly non-aromatic lowboiling distillate tar oil containing a'controlled amount of lowboilingv tar'acids.

Forthe'purposeof this specification, this rnigitureiof 2,774,716Patented Dec. 18, 1956 "ice boiling tar oils and a small amount of lowboiling tar acids shall be'termed solvent.

The solvent dissolves a major portion of the normally liquid tar andrejects from solution a small amount of the heavy pitch, Droplets ofthis rejected pitch become coated with the powdery solidsfrom the tarand settle out from the mixture in discrete agglomerated particles whichare recovered separately from the liquid tar. The solvent is strippedfrom the liquid tar and recycled. A significant feature of the presentinvention is the autogenous production of small amounts of solvent fromthe raw tar being treated. This feature permits continuous operation ofthe solids separation process without requiring the addition ofextrinsic solvent.

The low boiling tar oils which form the solvent are those low boilinghydrocarbons normally present in low temperature carbonization tars.Those low boiling tar oils represent less than about 5 percent of theweight of low temperature carbonization tars, and usually less thanabout 2 percent, depending upon the distillation techniques selected.The composition of the low boiling tar oil from a typical lowtemperature carbonization tar is presented in The Journal of theInstitute of Fuel, vol. XX, No. 113, April 1947, at page 111. As setforth in Table IV of the reference, the low boiling oil represents about2 percent of the total condensed liquid products from low temperaturecarbonization. Its composition is as follows (in percent):

Crude phenols 12' Saturated hydrocarbons (parafirns and naphthenes)-21.5 Olefinic hydrocarbons 34.5 Aromatic hydrocarbons 30 The exactcomposition of low boiling tar oils may vary somewhat from this typicalanalysis, but in all cases, low temperature carbonization tars arepredomiv nantly non-aromatic in contrast to the tars resulting from hightemperature carbonization. v

For the purpose of this invention I prefer to use as solventthe entirelow boiling tar distillate fraction having a boiling end pointtemperature below 200 C. but above 155 C. The initial boiling point ofthis fraction is generally about C., but may vary somewhat with'thenature of the tar from which the low boiling tar oil is derived, Whilethe entire fraction is preferred for ease of operation, nevertheless anynarrower fraction thereof may be employed; The low boiling tar oil mustin addition contain at least 2 percent by volume of tar acid in order tocomprise the solvent of this invention. Insome instances the solvent maybe a distillate fraction of the low temperature carbonization tar beingan end point in excess of-ZOO" C. In other instances the solvent may beablendof two or more distillate fractions ,of the tar.

I have discoveredthat a solvent fulfilling the specifications and suitedto the purposes of this invention can be made without thedeliberateuaddition of tar acids by recovering the.,e ntire distillatefraction from the low temperature carbonization tar which inherentlycontains the requisite quantity of tar acids. Thedis'tillation endtemperature may beselectedgso as to control the tar acids content quite.closely. Those fractionshaving-a distillation end point below about C.will contain virtually no tar acids whereas fractions having-adistillation end point of about 200 C. might contain 15 to, 30 percenttar acids. .;The solvent from low temperature carbonization tar, byitself, is not wholly miscible with a batch of raw tar; instead, thesolvent effects-a separation of a tacky nonfi-lterable pitch phasetogether with the particles of solid fuel-contained in the raw tar Theliquid portion of raw low temperature carbonization tars is wholly'miscible with tar acids by themselves; thus tar acids alone will notreject pitch from low temperature carbonization tars.

Nevertheless, if solids-laden tars are treated with low boiling tar oilscontaining the proper amount of tar acids for the particular raw tar, anoptimum quantity of pitch can be rejected from the raw tar to bindvirtually all the raw tar solids into readily filterablelargeragglomerated particles. Insufficient taracids in the treatingsolvent re.- sult in excessive pitch rejection and accompanyingnonfilterability. Excessive tar acids in the treating solvent result ininsufiicient pitch rejection and very little improvement in thefilterability of the raw tar.

The treating solvent should contain at least 2 and preferably less thanpercent by volume :of tar acids, with the balance comprising low boilingtar oils. The exact optimum concentration depends upon the raw tar beingtreated. Factors controlling the raw tar composition include the type ofcoal used .in the carbonization process, the nature of the carbonizationprocess, the technique of tar recovery and the fraction of solid fuelparticles found in the raw tar.

For a better understanding of my invention and its objects, referenceshould be had to the drawing which is a schematic representation of thepreferred embodiment of my invention.

In the drawing, 19 is a mixing vessel, 12 is a continuous filter, 14 isa solvent stripping column, 16 is a cooler, 18 is a condenser, and 20 isa solvent storage vessel.

Raw tar from a low temperature carbonization process is introducedcontinuously into the mixing vessel 19 through conduit 24. This raw tarcontains all the normally condensible overhead products from a lowtemperature carbonization process. In addition, the tar containsparticles of solid fuel which have been entrained in the vapors leavingthe carbonizing apparatus. These particles of solid fuel are coal orpartially devolatilized coal or a mixture of both, depending upon thecarbonizing apparatus and the starting coal. In the Disco carbonizationprocess, for example, tars are recovered containing up to weight percentof solid fuel particles, which are coal and partially devolatilizedfuel, with the coal particles predominating. In other carbonizationprocesses, the solid fuel particles in the tar are almost exclusivelypartially devolatilized fuel. These fines, of course, possess a high ashcomposition.

Solvent from vessel 20 is introduced into the mixing vessel 10 throughconduit 26. In general from 0.5 to 3.0 volumes of solvent are employedfor-each volume of raw tar. Preferably the solvent to raw tar ratio isabout 1:1.

Low solvent to tar ratios (i. e., less than 0.521) produce tacky,non-filterable pitch precipitates. There appears to be little advantagein increasing the solvent to tar ratio above 3:1.

in the mixing vessel 10, the intermixing of the solvent and the raw tarrejects a portion of the heavy pitch from the liquid phase. Droplets ofthis rejected, sticky pitch become coated with the powdery solids fromthe raw tar and settle out as discrete, non-tacky, agglomeratedparticles.

The mixture of agglomerated particles, tar and solvent passes from themixing vessel 10 through conduit 28 to a continuous filter 12. Theprecipitated pitch and agglomerated solids are collected as a filtercake which can be recovered and briquetted along with additional coal orchar to produce a solid fuel briquet. Alternatively, since the processof this invention is integrated with a low temperature carbonizationoperation, the filter cake can be returned to the carbonization vesselas a feed material. Since the filter cake contains some solvent and tarcomponents, it can be treated to permit recovery of the valuableliquids.

The filtrate from the continuous filter '12 is pumped Development of theDisco iProcess :of Low Temperature Carbonizationw'by C. E. Lesher,Mining Engineering 4, 287-99 (March 1952).

through conduit 30 to a stripping column where the solvent is distilledoverhead. Since the raw tar from the carbonization plant contains lowboiling tar oils, as well as low boiling tar acids, there are no solventmake-up problems associated with the present process. Each unit of rawtar being treated supplies a small amount of the low boiling tar oilsand tar acids which constitute the solvent. This autogenous solventproduction is sufiicient to compensate for any reasonable operatingsolvent losses in the system.

it is significant to note that the solvent and the filtered tar need notbe separated immediately. After the initial pitch rejection in theagitation stage, no further separation Will occur through the action ofthe solvent. The solvent (as the term is defined in this application) ofthis invention has no tendency to react continuously with the heavycomponents of the pitch. Not all the pitch contained in the raw tar isrejected by the solvent treatment of this invention. Much of the pitchremains in solution after mixing with the solvent and is recovered as avaluable ash-free product.

The composition of the solvent is controlled by regulating the operatingconditions of the stripping column which provides an overhead lowboiling tar oil product containing the desired tar acids concentration.The lowest boiling tar acid, phenol, has a normal boiling temperature ofl C. in its pure form. However, small amounts of phenols and other lowboiling tar acids form azeotropes and boil along with the low boilingtar oils at temperatures below the normal boiling point of the pure taracids. In fact I have found that filtrate fractions having a boiling endpoint temperature of about C. normally contain the proper amount of taracids required to treat some tars in the agglomerating step of theprocess. Thus the tar oil distillate fraction boiling below about 160 C.usually contains about 2 to 10 percent tar acids by volume resultingfrom the azeotropic character of the low boiling tar acids. For most rawtars, this 160 C..boiling end point fraction will be a suitable treatingsolvent. The discovery that this low boiling tar oil possesses tar acidazeotropes and can be used as treated solvent greatly simplifies thesolids removal process.

Solvents with high tar acids concentration precipitate less pitch fromthe raw tar and hence permit the recovery of slightly greater quantitiesof liquid product tar. However, when the tar acids concentration in thesolvent is too high, no pitch precipitation occurs and the objectionablesolids cannot be readily removed from the liquid tar.

The stripped solvent passes overhead from the stripping column 14through conduit 32 and condenser 18 to the solvent storage vessel 20. Aportion of the overhead solvent is returned .as reflux to the column 14through conduit 33. Any water in the solvent stream settles in vessel 20and is withdrawn through a conduit 34. The Water is combined withcarbonization plant liquor for treatment.

The bulk of the tar and pitch contained in the initial raw tar isrecoveredas a bottom product from the stripping column 14 throughconduit 36 and cooler 16. This tar is substantially free of any solidmaterial and can be processed easily in conventional distillation andextraction operations. The pitch contained in the bottom product fromthe stripping column, because it is essentially ash-free, finds manyvaluable uses, e. g., as a raw material .for the production of electrodecarbon.

'In order to commence commercial operation of a solids removal systemfor raw low temperature carbonization tars it is frequently impracticalto attempt to collect a supply of low boiling tar oils to be used assolvent. Accordingly for the initial operation of a rawtar solidsremoval system, commercial low boilingmaterials may be obtained whichwill serve the same purpose .as the solvent .described herein. Forexample, .benzene alone is an excellent material for treating .raw tarto effect the desired agglomeration of pitch together with dryagglomerated solid particles. The benzene would circulate through thesystem until replaced by the autogenous solvent of this invention.Foreign solvents such as benzene migh't be selected which would efiectthe proper amount of pitch rejection to bind the solids in the raw tar.Benzene, of course, is an unsatisfactory solvent for continued solidsremoval use because of its expense and because excessive solvent and tarare retained in the filter cakes resulting from operation with benzene.

The pitch rejection step of the present invention is quite effective incausing a particle size accretion of the solids contained in raw tar. Infact it is possible to recover solids-free tar simply by decantation ofthe liquid remaining above the rejected pitch and solids. Such a system,of course, could be employed in combination with a vacuum drying of thesettled solids to recover the liquids associated with the solids. Acentrifuging step can be substituted for the filtration step describedin connection with the drawing. The presence of the solvent tends toreduce the viscosity of the tar and the increased size of the solidsreadily permits their centrifugal separation. 7

Example The present process was employed to remove solids from a raw tarderived from the low temperature carbonization of Pittsburgh seam coal.

The. raw tar contained about 16 percent by weight of finely dividedsolids which had a differentialscreen analysis as follows:

Sieve size: Weight percent on 48 0.0 100 0.6

325 48.6 through -325 50.2

This tar was unfilterable in its raw state. However, after agitatingthis raw tar at 24 C. with an equal volume of solvent composed of lowboiling tar oils having a boiling end point of 160 C. and containing 6percent by volume of low boiling tar acids, the mixture was filteredwithout difiiculty.

The liquid filtrate contained virtually no ash and the recovered solventwas a greater volume than that employed in the agitation stage. Thesolvent stripped tar was about 78 percent by weight of the initial rawtar. The differential screen analysis of the filter cake solids was asfollows:

1 If desired, filtration rates for the tar can be increased even furtherby adding small quantities of coarse char to the raw tar before thesolvent treatment.

As seen from the example, the present process can be operated atordinary room temperatures. Slightly higher temperatures permitoperation of the process with a reduced tar acids concentration in thesolvent.

According to the provisions of the Patent Statutes, I have explained theprinciple, preferred construction, and mode of operation of my inventionand have illustrated and described what I now consider to represent itsbest embodiment. However, I desire to have it understood that, withinthe scope of the appended claims, the invention may be practicedotherwise than as specifically illustrated and described.

I claim: e 1 -f 1.A process for'removing solids consisting essentiallyof finely divided solid particles from the class of coal and partiallydevolatilized coal from raw low tempera ture carbonization tar. whichcomprises mixing the raw tar with 0.5 to 3.0 volumespf a predominantlynonaromatic low temperature carbonization tar distillate fractioncontaining at least 2 percent by volume of tar acids, separatingrejected pitch and agglomerated solids, and recovering the liquids.

2. In the removal from raw low temperature carbonization tar thoseparticles of solid fuel which become entrained in the tar during thecarbonization and tar recovery processes, the improvement whichcomprises mixing said raw tar with 0.5 to 3.0 volumes of a solventcomprising a predominantly nonaromatic distillate fraction from lowtemperature carbonization tar containing at least 2 percent tar acids byvolume, separating from the resulting mixture rejected pitch andagglomerated solids, and recovering the liquids.

. 3. A process for removing solids consisting essentially of finelydivided particles from the class of coal and partially devolatilizedcoal from raw low temperature carbonization tar which comprises mixingthe raw tar with 0.5 to 3.0 volumes of a predominantly nonaromatic lowtemperature carbonization tar distillate fraction having a boiling endpoint below 200? C. and containing 2 to 20 percent byvoilume of taracids, separating rejected pitch and agglomerated solids and recoveringthe liquids.

4. A process for removing solids consisting essentially of finelydivided particles from the class of coal and partially devolatilizedcoal from raw low temperature carbonization tar which comprises mixingraw low temperature carbonization tar with 0.5 to 3.0 volumes of apredominantly nonaromatic low temperature carbonization tar distillatefraction having a boiling end point below 200 C. and containing 2 to 20percent by volume of tar acids, filtering the resulting mixture andrecovering the filtrate;

5. A process for removing solids consisting essentially of finelydivided particles from the class of coal and partially devolatilizedcoal from raw low temperature carbonization tar which comprises addingto the raw tar 0.5 to 3.0 volumes of a predominantly nonaromatic lowtemperature tar distillate fraction having a boiling end point below 200C. and containing 2 to 20 percent by volume of tar acids, separatingrejected pitch and agglomerated solids from the mixture, recovering theliquids, separately recovering from said-liquids a distillate fractionhaving a boiling end point below 200 C. and containing 2 to 20 percentby volume of tar acids, and separately recovering the remaining liquids.

6. A process for recovering ash-free tar and pitch from raw lowtemperature carbonization tar which comprises continuously agitatingeach volume of tar along with 0.5 to 3.0 volumes of solvent comprising apredominantly nonaromatic low temperature carbonization tar distillatefraction having a boiling end point below 200 C. and containing 2 to 20percent tar acids by volume, continuously filtering the mixture of tarand solvent, recovering the filtrate, recovering from said filtrate adistillate frac- -tion having a boiling end point below 200 C. andcontaining 2 to 20 percent by weight of tar acids, separately recoveringthe remaining filtrate, and recovering said distillate fraction assolvent.

7. A continuous process for removing solids consisting of finely dividedparticles from the class of coal and partially devolatilized coal fromraw low temperature carbonization tar which comprises continuouslymixing each volume of said tar with 0.5 to 3.0 volumes of solventcomprising a predominantly nonaromatic low temperature carbonization tardistillate fraction having a boiling end point below 200 C. andcontaining 2 to 20 percent tar acids by volume, continuously filteringthe mixture of tar and solvent, recovering the filtrate, recovering fromsaid filtrate a distillate fraction having a boiling end pointtemperature below 200 C., said temperature being selected so that thedistillate fraction will contain 2 to 20 percent by volume of tar acids,separately recovering the remaining filtrate and recovering saiddistillate fraction to be used as solvent in a cyclic manner.

8. A process for removing solids consisting essentially of finelydivided particles from the class of coal and partially devolatilizedcoal from the raw tar produced by the low temperature carbonization ofhigh volatile bituminous coal which comprises mixing each volume of saidtar with 0.5 to 30 volumes of solvent comprising a predominantlynonaromatic distillate fraction of low temperature carbonization tarhaving a boiling end point below 200 C. and containing 2 to 20 percenttar acids by volume, filtering the mixture of tar and solvent,recovering the filtrate, recovering as solvent from said filtrate thedistillate fraction having a boiling end point below 200 C. andseparately recovering the remaining filtrate.

9. A continuous process for removing solids consisting essentially offinely divided particles from the class of coal and partiallydevolatilized coal from the raw tar produced by the low temperaturecarbonization of high volatile bituminous coal which comprises mixingeach volume of said tar with 0.5 to 3.0 volumes of solvent comprising apredominantly nonaromatic distillate fraction of low temperaturecarbonization tar having a boiling end point below 200 C. and containing2 to 20 percent tar acids by volume, continuous-1y filtering the mixtureof tar and solvent, recovering the filtrate, recovering as solvent fromsaid filtrate a distillate fraction having a boiling end point below 200C., separately recovering the remaining filtrate, and recycling saiddistillate fraction to be used as solvent in a cyclic manner.

10. The process of claim 9 in which the high volatile bituminous coal isPittsburgh seam coal.

11. In the removal from raw low temperature carbonization tar of thoseparticles of solid fuel which become entrained in the tar during thecarbonization and tar recovery processes, the improvement whichcomprises mixing said raw tar with 0.5 to 3.0 volumes of a solventcomprising a predominantly nonaromatic distillate fraction from lowtemperature carbonization tar having an end boiling point below 200 C.and containing at least 2 percent by volume of tar acids, separatingfrom the resulting mixture rejected pitch and agglomerated solids andrecovering the liquids.

References Cited in the file of this patent UNITED STATES PATENTS1,960,679 Parkhurst May 29, 1934 2,631,982 Donegan Mar. 17, 1953 FOREIGNPATENTS 590,748 France Mar. 25, 1925

1. A PROCESS FOR REMOVING SOLIDS CONSISTING ESSENTIALLY OF FINELYDIVIDED SOLID PARTICLES FROM THE CLASS OF COAL AND PARTIALLYDEVOLATILIZED COAL FROM A RAW LOW TEMPERATURE CARBONIZATION TAR WHICHCOMPRISES MIXING THE RAW TAR WITH 0.5 TO 3.0 VOLUMES OF A PREDOMINANTLYNONAROMATIC LOW TEMPERATURE CARBONIZATION TAR DISTILLATE FRACTIONCONTAINING AT LEAST 2 PERCENT BY VOLUME OF TAR ACIDS, SEPARATINGREJECTED PITCH AND AGGLOMERATED SOLIDS, AND RECOVERING THE LIQUIDS.